set retain policy to destroy for resources created by PR deploy previews

### Environment information

```plain text
n/a
```

### Describe the feature

Customers have the option to [enable previews for pull requests](https://docs.aws.amazon.com/amplify/latest/userguide/pr-previews.html) against their connected repository. This enables folks to create a new backend for each pull request, which is treated as a branch deployment with a special "pr" identifier. When pull requests are closed or otherwise merged, resources should be destroyed.

When backends are created from PR deploy previews, resources within the stack should have their retain policy set to DESTROY, similar to sandbox deployments.

### Use case

n/a

No files, tests, or entry points are named. Start by locating the code that creates backends for pull-request deploy previews and comparing its resource-retention handling with sandbox deployments. Done means resources in PR preview stacks use DESTROY and are removed when the pull request is closed or merged.

Written by the indexing model from the issue text.